1. Field of the Invention
The invention relates in general to a voltage supplying apparatus, and more particularly to a voltage supplying apparatus preventing the breakdown of the electronic circuit coupled thereto due to the process deviation of a voltage-drop device.
2. Description of the Related Art
Ordinary electronic circuits such as a voltage control oscillator (VCO) must use a voltage supplier to provide a stable operating voltage Vcc. Referring to FIG. 1, a circuit diagram of a conventional voltage control oscillator coupled with a voltage supplier is shown. The conventional voltage supplier 100 includes a voltage generator 110 and a regulator 120. The voltage generator 110 is for providing a constant voltage Vs, i.e., a band gap reference voltage. The regulator 120 includes an amplifier 122 and a P-typed metal oxide semiconductor (PMOS) transistor P1. The amplifier 122 has its negative input terminal (−) coupled with the voltage generator 110 for receiving the constant voltage Vs and has its output terminal coupled with the gate electrode of the transistor P1. The resistor R2 and the resistor R1 are serially connected to the drain electrode of the transistor P1. The amplifier 122 has its positive input terminal (+) connected to the joint of the resistor R2 and the resistor R1 to form a feedback circuit. The drain electrode of the transistor P1 is for outputting an output voltage Vo. Therefore, the relation between the output voltage Vo and input voltage Vs is expressed as Vo=Vs*(R1+R2)/R1 according to the feedback circuit inside the regulator 120.
However, during wafer manufacturing process, the voltage-drop device included in VCO 130, a resistor R3 or a transistor N1 (or N3) for instance, normally requires a process of multi-layer masks. During the lithography manufacturing process, the resistor or the transistor located in the middle part of the exposure region of the same wafer can be manufactured with a higher accuracy than otherwise. That is to say, for transistors and resistors located in the middle part of the exposure region, their respective threshold voltage and resistance value are more accurate than those located in other parts thereof. The exposure region located in the edges of a mask is likely to have deviation in resistance value or threshold voltage due to the diffraction or reflection of the light. Besides, during the ion doping process, the density of the doping ions may vary with their locations on the wafer and thus the features of the devices on the wafer may also vary with their locations on the wafer. Normally, the deviation in resistance value or threshold voltage generated due to the process deviation is approximately ±20%. Since the voltage-drop devices have the process deviation, deviation in an input current Io of the VCO130 occurs. Moreover, when the input current Io is too weak, the normal operation of VCO130 will even be affected.